In a conventional VTR, a capstan motor is used for controlling the advancing speed of the tape past a transducer. Nowadays, the VTR tends towards a multifunctional and high quality system, thus the rotation speed of the capstan motor should be exactly measured for controlling the system over a wide speed range such as a still picture, a slow motion, and fast forward and reverse speed replay.
A conventional motor speed measuring method has been executed as follows. In FIG. 1, signal pulse FG is obtained from a frequency pulse generator attached to a capstan motor. The period of a reference wave Tc between a point b delayed by a predetermined time from the falling edge a of pulse FG and a next delay point d is counted. From the falling edge a of the pulse, a bias time Tb for detecting speed is then set. From the end point f of the bias time Tb, counting is executed to the delay point d to determine the count value Ts as a speed measurement. If the rotation speed becomes faster, the period T of the FG pulse becomes short, thereby decreasing the count value Ts. On the contrary, if the speed becomes slower, the period T becomes longer to increase the count value Ts. Td denotes the total time for determining the rotation speed.
Accordingly, if the speed becomes much faster to make T&lt;Tb, the next delayed point d of the signal pulse FG precedes the point f, thus the value of speed measurement Ts becomes "0". On the other hand, if the speed becomes much slower to make T&gt;Tb+Tb, the next delayed point d of the signal pulse FG becomes above the value of 2.sup.n -1 which is the maximum set value for the count value Ts, and the value of the speed measurement has the value of 2.sup.n -1 regardless of the speed. Thus, in high speed and low speed region, the errors in the speed measurement value are generated. Accordingly, there arises a problem in that the speed detecting bias time Tb should be varied according to the speed. That is, the speed detecting bias time Tb should be controlled to be shortened in a high speed mode, and to be lengthened in a low speed mode. But, in order to vary the time Tb according to a speed mode, a complicated algorithm is required, so that the speed measuring program becomes very complicated.